1. Field of the Invention
A fluid heat exchanger assembly for cooling an electronic device.
2. Description of the Prior Art
Research activities have focused on developing assemblies to efficiently dissipate heat from electronic devices that are highly concentrated heat sources, such as microprocessors and computer chips. These electronic devices typically have power densities in the range of about 5 to 35 W/cm2 and relatively small available space for placement of fans, heat exchangers, heat sink assemblies and the like. However, these electronic devices are increasingly being miniaturized and designed to achieve increased computing speeds that generate heat up to 200 W/cm2.
Heat exchangers and heat sink assemblies have been used that apply natural or forced convection cooling methods to cool the electronic devices. These heat exchangers typically use air to directly remove heat from the electronic devices. However, air has a relatively low heat capacity. Such heat sink assemblies are suitable for removing heat from relatively low power heat sources with power density in the range of 5 to 15 W/cm2. The increased computing speeds result in corresponding increases in the power density of the electronic devices in the order of 20 to 35 W/cm2 thus requiring more effective heat sink assemblies.
In response to the increased heat to be dissipated, liquid-cooled units called LCUs employing a cold plate in conjunction with high heat capacity fluids, like water and water-glycol solutions, have been used to remove heat from these types of high power density heat sources. One type of LCU circulates the cooling liquid so that the liquid removes heat from the heat source, like a computer chip, affixed to the cold plate, and is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger and an air moving device such as a fan or a blower. These types of LCUs are characterized as indirect cooling units since they remove heat from the heat source indirectly by a secondary working fluid, generally a single-phase liquid, which first removes heat from the heat source and then dissipates it into the air stream flowing through the remotely located liquid-to-air heat exchanger.
As computing speeds continue to increase even more dramatically, the corresponding power densities of the devices rise up to 200 W/cm2. The constraints of the miniaturization coupled with high heat flux generated by such devices call for extremely efficient, compact, and reliable thermosiphon cooling units called TCUs. A typical TCU absorbs heat generated by the electronic device by vaporizing the captive working fluid on a boiler plate of the unit. The boiling of the working fluid constitutes a phase change from liquid-to-vapor state and as such the working fluid of the TCU is considered to be a two-phase fluid. The vapor generated during boiling of the working fluid is then transferred to an air-cooled condenser, in close proximity to the boiler plate, where it is liquefied by the process of film condensation over the condensing surface of the TCU. The heat is rejected into an air stream flowing over a finned external surface of the condenser. The condensed liquid is returned back to the boiler plate by gravity to continue the boiling-condensing cycle. These TCUs require boiling and condensing processes to occur in close proximity to each other thereby imposing conflicting thermal conditions in a relatively small volume. This poses significant challenges to the process of optimizing the TCU performance.
Illustrative examples of the prior art are shown in U.S. Pat. Nos. 6,360,814; 5,998,863; 5,239,200 and 4,953,634. The '814 patent discloses a TCU having a boiler plate with rectangular shaped fins. The rectangular shaped fins dissipate heat from the electronic device. The '863 patent discloses another TCU having a boiler plate with fins for dissipating heat. The fins are transverse to the cooling fluid flow and therefore restrict the flow of the cooling fluid and divide the chamber into discrete compartments. Such a design reduces the amount of heat that the TCU is capable of dissipating. In the '200 patent, all of the flow of coolant is tortuous and in the '634 patent, the cross sectional area of the flow path is decreased and the fins are parallel to the flow path. Another TCU is disclosed in WO 02/092897 having a boiler plate with various shaped fins. The known heat exchangers or heat sinks have upper and lower walls extending between an inlet and an outlet for establishing a direction of flow from the inlet to the outlet. The heat sinks often include projections from the walls to enhance heat transfer. However, the flow is normally parallel to the projections and straight whereby the contact between the flow and the projections is determined by the rate of flow. In order to increase the dwell time for the fluid to be in contact with the projections to increase the heat transfer rate, a zigzag flow path has been utilized as in the '200 patent.